Flyback converter topologies which include a transformer providing galvanic isolation between the input and the output of the converter are commonly used in switched-mode power supplies (SMPS) of both AC/DC and DC/DC kind. Flyback topologies offer a good ratio of system costs to device performance over a wide output power range from 1 W to 100 W and more. However, when a transition to higher power classes is made, a limitation of the maximal efficiency to be achieved becomes more significant in comparison to other topologies and corresponding driving methods of switched mode power supplies. There are multiple parts in a flyback type appliance which affect the overall efficiency of the appliance in a limiting manner due to their respective power losses.
The voltage conversion in SMPS based on flyback topologies is based on an operating scheme basically involving two steps. In a first step, a power switch provided in the circuit including a primary side of the transformer is closed and energy provided at an input of the converter is stored in the magnetic field of the transformer. In a second step, the power switch on the primary side of the transformer is opened and a power switch on the secondary side of the transformer is closed, whereby the energy stored in the magnetic field of the transformer drives a demagnetization current through a secondary side of the transformer until the transformer is demagnetized. The demagnetization current flows through a rectifying diode in the circuit including the secondary side of the transformer and charges an output capacitor which is configured to provide an output voltage.
During the conduction phase of the demagnetization current through the rectifying diode, the diode forward voltage drop is responsible for a power loss. The demagnetization currents rise drastically for higher powers and in appliances with small output voltages and therefore the corresponding power loss in the rectifying diode also increases.
The problem of the power loss in a diode inherent in its forward voltage drop can be tackled in various ways. One possible approach is the synchronous rectification scheme, according to which, on the secondary side of the transformer, the function of the rectifying diode in conducting state is replaced by a power switch which is switched in predefined time intervals and is driven in sync with the power switch on the primary side of the converter. The power switch on the secondary side of the transformer can avoid the forward voltage drop of the rectifying diode in conducting operation and the power loss connected therewith.